This invention relates to a correlation device adapted to be used for spectrum spread communication which is utilized for radio controlling of a crane or the like, local communication, crosstalk-proof communication, and the like, and more particularly to a correlation device of a matched filter system.
Spectrum spread communication has been conventionally studied and partially put into practice in various fields such as crosstalk-proof communication, remoter control, local area network and the like.
Conventionally, a direct spread (DS) system is used as one of systems for spectrum communication. In the DS system, it is required to attain synchronism between a transmission side and a receiving side in order to decode a signal received by the system.
Normally, correlation decode is carried out for decoding a spread signal and a delayed lock loop (DLL) system is used as one of systems used for this purpose. Unfortunately, the DLL system has disadvantages relating to time for the supplementing of synchronism and the stable operation. In view of the foregoing, attention is paid to a matched filter (adaptive filter) system recently.
For matched filtering are used a system of using a surface acoustic wave (SAW), a system of using a digital circuit and the like. The matched filter system exhibits advantages of permitting the high-speed synchronism, the stable operation and the like to be accomplished. However, the SAW system has a disadvantage of causing the attenuation of a signal during the transmission of the signal and the like.
A correlation operation using the matched filter is expressed by the following formula; ##EQU1## wherein d.sub.k indicates an input signal, P.sub.(k+n) indicates each of pits of a spread code, and N indicates a length of the spread code.
Thus, the above formula can be realized by carrying out the operation for summing products by the number of times N equal to the length of the spread code.
FIG. 3 shows a conventional matched filter wherein the above formula is realized by a digital circuit.
In FIG. 3, reference numeral 301 designates a shift register, 302 is each of multipliers and 303 is an adder. An input signal V.sub.i received from a transmission side is a signal formed by spreading a base band signal by means of a spread code and is supplied to the sift register 301 in order. Data d.sub.O to d.sub.N stored in the shift register 301 each are multiplied by a spread signal identical with that on the transmission side. More particularly, the data d.sub.O to d.sub.N are multiplied by pits P.sub.O to P.sub.N of the spread code by means of the multipliers 302, respectively. Then, they are subject to addition in the adder 303, so that a correlation output signal V.sub.O corresponding to the base band signal may be obtained.
The matched filter constructed as described above permits the high-speed synchronism to be carried out and the operation to be stabilized.
However, the matched filter includes N multipliers and the adder includes N taps, so that the matched filter is highly complicated in circuit construction, resulting in being expensive. Also, the matched filter of the digital system causes the operation speed of the multipliers to be restricted, to thereby fail to increase the processing speed.